Re: build error: sparsemem + SLOB

[Randy Dunlap <randy.dunlap@oracle.com>]

On Tue, 21 Nov 2006 14:57:23 +0900 Yasunori Goto wrote:

> > On Mon, Nov 20, 2006 at 05:28:24PM +0000, Hugh Dickins wrote:
> > > On Mon, 20 Nov 2006, Christoph Lameter wrote:
> > > > 
> > > > As far as I can tell SLOB is fundamentally racy since it does not support 
> > > > SLAB_DESTROY_BY_RCU correctly. F.e. The constructor for the anon_vma will 
> > > > be called on alloc without regard for RCU, we free an item and reuse it 
> > > > without regard to RCU. This can potentially mess up the anon_vma locking 
> > > > state while we access it.
> > > 
> > > Good find!
> > > 
> > > > Is SLOB used at all or have we been lucky so far?
> > >
> > > Lucky so far.  Well, we'd actually have to be quite unlucky to ever
> > > see what page_lock_anon_vma/SLAB_DESTROY_BY_RCU are guarding against.
> > >
> > > But you're absolutely right that users should not be exposed to such
> > > unsafety.  I'd say SLOB should be disallowed if SMP.
> > 
> > SLOB is an O(N) allocator and is pretty poorly suited to running on
> > anything like a modern desktop. Disallowing if SMP is probably
> > reasonable, as even machines with multicore ARM or MIPS will probably
> > have enough memory to make SLOB a bit painful.
> 
> Ok. It's simple. This is fix.
> 
> ----
> 
> This is to disallow to make SLOB with SMP or SPARSEMEM.
> This avoids latent troubles of SLOB with SLAB_DESTROY_BY_RCU.
> And fix compile error.
> 
> This patch is for 2.6.19-rc5-mm2.
> 
> 
> Signed-off-by: Yasunori Goto <y-goto@jp.fujitsu.com>

Acked-by: Randy Dunlap <randy.dunlap@oracle.com>

Thanks.

> ----
>  init/Kconfig |    2 +-
>  1 files changed, 1 insertion(+), 1 deletion(-)
> 
> Index: 19-rc5-mm2/init/Kconfig
> ===================================================================
> --- 19-rc5-mm2.orig/init/Kconfig	2006-11-21 13:41:31.000000000 +0900
> +++ 19-rc5-mm2/init/Kconfig	2006-11-21 14:21:31.000000000 +0900
> @@ -465,7 +465,7 @@ config SHMEM
>  
>  config SLAB
>  	default y
> -	bool "Use full SLAB allocator" if EMBEDDED
> +	bool "Use full SLAB allocator" if (EMBEDDED && !SMP && !SPARSEMEM)
>  	help
>  	  Disabling this replaces the advanced SLAB allocator and
>  	  kmalloc support with the drastically simpler SLOB allocator.
> 
> 
> -- 

---
~Randy

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